1. Field of the Invention
This invention relates to the field of data transmission and particularly to data transmission using a secondary channel FSK modem.
2. Background Art
Communication of data information on a voice network between a sending and receiving station is typically accomplished with a modem. A modem (modulator/demodulator) transforms digital information by modulating it according to a well known modulating scheme. This modulated data is transmitted to a receiving modem where it is demodulated to produce the original information.
There are several types of modulating schemes which may be utilized in digital transmission, such as frequency shift key (FSK), differential phase shift key (DPSK), quadrature amplitude modulation (QAM), etc.
For high-speed data transmission, dedicated unidirectional, half duplex modems are utilized. These modems are typically QAM modems with transmission rates of 9600 bps or greater, with large bandwidth overhead requirements. Although typically unidirectional, it is often desired to provide a secondary channel to provide information with the primary transmission. This secondary channel is typically an FSK-type transmission channel and is confined to a narrowband in the voice bandwidth. The transmission rate is substantially lower than the primary channel, often on the order of 5-110 bps.
In the prior art, a discrete secondary channel is often added to the primary QAM channel. A disadvantage of such discrete secondary channel FSK modems is high cost, poor performance and limited functions. For example, due to cost and board size limitations, the group delay equalization is not implemented with the band split filters. This results in substantial distortion to the QAM signal and degrades QAM modem performance to an unacceptable level for data rates over 9.6 kbps. In addition, the separating of two carrier frequencies in prior art FSK modems is on the order of 60 Hz. This requires a wider bandwidth for transmission, lowering efficiency of bandwidth utilization and prohibiting its use with 19.2 kbps QAM modems. Therefore, it is an object of the present invention to provide sufficient channel separation between QAM and FSK signals while generating minimum noise and distortion on the QAM signal.
It is another object of the present invention to provide a secondary channel FSK modem with separation of two carrier frequencies of 40 Hz.
It is yet another object of the present invention to provide a secondary channel FSK modem which constrains the FSK signal in a frequency band from 300 Hz to 400 Hz.
It is still another object of the present invention to provide a secondary channel FSK modem which facilitates a high-performance FSK detection, i.e., a bit-error rate of 10.sup.-5 is achieved with a 4 dB signal-to-noise ratio when the FSK signal is received at -45 dBm.
It is yet another object of the present invention to provide a fully integrated secondary channel FSK modem.